The present embodiments relate to a method for digital amplitude control and digital phase control of a high-frequency signal.
Controlled high-frequency signals are used in a plurality of technical systems. Typical application examples are found, for example, in communications technology and medical technology. In the medical field, for example, modulated high-frequency signals are used in magnetic resonance systems. In order to generate an image using a magnetic resonance tomography method, defined high-frequency pulses are to be irradiated into a body or into a body part of a patient to be examined. The patient is located in a precisely defined magnetic field. The nuclear spins of the atoms in the examination object are thereby excited (e.g., tilted in a defined manner by a flip angle). The resulting emitted signals of the nuclear spins are captured and acquired as raw data, from which the desired magnetic resonance images may be generated. An extremely wide variety of high-frequency pulse sequences may be emitted for various examinations. Each individual high-frequency pulse is to have a precisely defined temporal length, amplitude and shape in order to achieve a specific effect. The parameters used for this purpose may be specified in the form of a digital data stream that is mixed with a mixed frequency MF, thereby ultimately producing the high-frequency signal that has been modulated in the desired manner (e.g., the required series of high-frequency pulses).
High-frequency signals change as a function of the load. The load generates a complex reflection factor, providing that part of the power that is delivered by the amplifier is reflected and phase rotated. For the purpose of control, both the signal from the amplifier to the load and the reflected signal are therefore to be taken into consideration.
The load in a magnetic resonance tomography system is dependent on the patient who is currently being examined and on the body part that is currently being examined. Before starting an examination, adjustment pulses, by which the power required in order to achieve a specified flip angle distribution for the respective examination situation may be specified in advance, may be emitted.
The load situation may change due to movement of the patient during the examination. The behavior of some components may change during the examination period due to temperature effects.
The amplitude and the phase of the high-frequency signal may be controlled during the examination.
DE 102 54 660 B4 describes a method where a feedback signal is demodulated in order to obtain a D.C. voltage signal, by which the amplitude of the high-frequency signal is controlled. A phase comparator is used to obtain a difference signal, using which a phase shifter is activated in order to control the phase of the high-frequency signal.
Digitization of such a method is disadvantageously problematic, as jumps may occur in the range of the phase control and seriously corrupt the control signal.